1. Field of the Invention
The present invention relates to a hub ring and a supporting plate for a filter and methods for manufacturing these members.
2. Description of the Related Art
Disc-type filters are known as filters for filtering a polymer, and for example, a so-called leaf disc-type filter and a three-layer type filter wherein disc-type filter materials are disposed on and under a retainer for forming a single filter element are well known. Such filters are usually used by stacking a plurality of filter elements by a number corresponding to a capacity of filtration to be required (flow rate of a molten polymer to be filtered). In the case of use of stacked filters, because it is necessary to seal a polymer after filtration from a polymer before filtration, a hub ring is provided on the inner circumference of each filter (as needed, it may be provided on the outer circumference of each filter). Such a hub ring seals polymers after and before filtration from each other and ensures a path for the polymer after filtration, as described above, as well as maintaining a positional relationship between respective stacked filters (for example, interval of filters).
As a typical structure of conventional filters, for example, a structure shown in FIG. 22 is known. FIG. 22 illustrates a part of an inner circumferential portion of a leaf disc-type filter 101. A filter medium 104 is provided on each side of a retainer 102 constructed from, for example, a mesh, via a filter medium supporting material 103 constructed from a perforated plate (for example, a punching metal). A hub ring 105 is provided on the circumference of filter 101, and a plurality of small holes 106 are provided in the hub ring 105. A molten polymer 107 flows into filter media 104 from upper and lower sides, and the polymer 107 filtered by the filter media. 104 passes through a portion of retainer 102 and holes 106 of hub ring 105 to be gathered .Iadd.at .Iaddend.a central portion, as shown by the arrows in FIG. 22.
In a case where a plurality of filters 101 are stacked, a predetermined number of filters 101 are fitted and stacked around a tubular supporting pole 108, and the gap between filter media 104 of adjacent filters 101 is set based on the dimension of the portion of hub ring 105. Respective hub rings 105 of stacked filters 101 are pressed from .Iadd.an .Iaddend.upper or .Iadd.a .Iaddend.lower side by a great fastening pressure capable of sealing polymers after and before filtration from each other, for example, a pressure of 5-10 tons.
In such a conventional structure, there remain the following problems.
Firstly, although a number of small holes 106 are opened in hub ring 105 for passing a polymer after filtration, the opening ratio thereof is about 20% at highest, and the small opening ratio causes a large pressure loss generated at this portion. Although it may be considered to enlarge the diameter of the holes in order to increase the opening ratio, the enlargement of the hole diameter is restricted to some extent by the above-described great fastening pressure.
Moreover, in the above structure defining small holes, in the upstream side of the polymer flow, the portions around the small holes 106 are likely to create a large dead space against the polymer flow. Such a dead space causes residence of polymer, and the residence of polymer causes degradation of the polymer and flowing-out of the degraded polymer, and therefore it is desired to suppress such a dead space as small as possible.
Furthermore, since the small holes 106 must be processed in the radial direction of hub ring 105, the processing is relatively difficult and a long time is required for the processing. Therefore, the cost for manufacturing the hub ring 105 is relatively expensive.
Still further, for the retainer 102, a relatively high strength particularly against deformation of the retainer in the thickness direction thereof, for example, a pressure resistance of about or higher than 200 kg/cm.sup.2, is usually required in order to maintain the form of the filter to an acceptable form and in order to ensure the polymer path in the radial direction of the filter at the position of the retainer. Even by the mesh type retainer 102, such a high strength for a high pressure resistance can be satisfied. In the mesh type retainer 102, however, particularly there are the following problems with respect to pressure loss and polymer residence.
Namely, in the mesh type retainer 102, because wires extending in the circumferential direction of the filter while waving (usually, the wire has a diameter corresponding to 1/2 of the thickness of the retainer 102 or a diameter slightly greater than that value) cause a large resistance against the polymer flow in the radial direction of the filter, it is difficult to suppress the pressure loss at the portion of the retainer 102, ultimately the pressure loss of the whole of the filter 101, to a small value.
Further, in ..the.!. portion of the a retainer 102, the polymer flows toward .Iadd.the .Iaddend.radially inner direction mainly along the wires extending in the radial direction of the filter. These wires extending in the radial direction of the filter form a dead volume against a polymer path rather than forming a polymer path, and therefore, the polymer flow has a poor directivity in the radial direction of the filter and it may be difficult to achieve a smooth polymer flow. The difficulty of a smooth polymer flow causes a poor chance of polymer mixing. Therefore, there is a latent problem that the polymer mixing effect (also called "static mixer effect") is small. Furthermore, there is a fear causing a problem of polymer residence. Also with the wires extending in the circumferential direction of the filter, there are similar problems because they form a dead volume against the polymer flow.